1. Field of the Invention
This present invention relates generally to a power conversion circuit and more particularly to a multiphase switching power converter.
2. Description of the Related Art
A typical power conversion circuit (e.g., a switching power converter) receives an input voltage and an input current and modifies the input voltage, the input current or both the input voltage and the input current to produce an output voltage and an output current. For example, a DC-to-DC converter receives input power from a DC voltage source at one voltage level and outputs a desired DC output voltage (typically, a regulated DC output voltage) at another level. A converter that includes a feedback loop to regulate one or more output parameters (e.g., voltage, current, etc.) is often referred to as a regulator. One embodiment of a converter is a switching converter that uses one or more switches to alternately connect and disconnect the voltage source to circuits that drive the output. The duty cycle of the switching is used to control the output voltage. The switching is typically controlled by a Pulse-Width Modulation (PWM) circuit.
The advancement of the microprocessor integrated circuit into the gigahertz frequency band of operation has led to the use of DC-to-DC converters that can operate in the multiphase mode. State-of-the-art processors are now operating with a core voltage ranging from 1.4 volts to 1.8 volts and with a core current in the range of 30 to 75 amperes. The continuous inductor current rating sets a typical limitation in output current that can be delivered by a single-phase converter. At normal operating frequencies, this current typically ranges between 2 and 20 amperes. Under these assumptions, a processor core needing 60 amperes requires a converter with four or more phases.
In a multi-phase switching converter, a PWM circuit provides a variable duty cycle signal to control the switching for each channel. The PWM signals are synchronous with different phases for each channel, thereby allowing each channel to be switched on at a different time. The multiple phases increase the output ripple frequency above the fundamental channel switching frequency and reduce the input ripple current, thereby significantly reducing the sizes of input capacitors and output capacitors, which are often large and expensive. Stress and heat on the components are also reduced because the output current is distributed among the multiple channels.
The DC current through each inductor is responsive to the duty cycle of its PWM signal and to the value of its voltage source. Each inductor has a current limit. Typically, more PWM circuits are used when more output current is desired. The output terminals of all the inductors from the PWM circuits are electrically connected to provide a single output of the power conversion circuit.
Since the output terminals of all the inductors are tied together, the conductors have substantially identical output voltages. The input terminal of each inductor has a rectangular wave voltage signal, which is derived by switching the input terminal between the voltage source and ground. The duty cycles of the rectangular wave voltage signals of respective channels are affected by variations in the respective PWM circuits and switches (e.g., design tolerances, offsets, and timing variations). For example, a slight difference in the duty cycle can produce a significant difference in the DC current through the inductor in each channel.
High efficiency power conversion circuits typically use inductors with low core loss (e.g., ferrite inductors). When the peak design current is exceeded (i.e., saturation), the inductance of ferrite core material collapses abruptly which results in an abrupt increase in inductor ripple current and output voltage ripple. Thus, it is important to keep the inductor core from saturating.
Forced current sharing is used to cause all the channels to contribute substantially identical currents to the output. Forced current sharing prevents an inductor in one of the channels from saturating. Prior art systems sense the current in each channel and adjust the respective duty cycles to produce the same current for each channel. Current sensing decreases the efficiency of the power conversion circuit because power is dissipated by a sensing resistor. Further, current sensing requires an undesirable ripple voltage across the sensing resistor in order to work properly. Other prior art systems employ costly precision design and trimming in an attempt to achieve accurate current sharing without sensing resistors. Typically, phase current mismatches are on the order of 30 percent or greater when employing precision duty cycle matched converters, necessitating the use of significantly higher current MOSFETs and inductors.
In a typical multiphase converter, the frequency of each phase is identical and the phase relationship between the various phases is adjusted to produce phase symmetry in the context of 0 to 360 degrees for one cycle. The typical phase relationship is 360 degrees divided by the number of phases used (e.g., in a two-phase converter, the phases are 180 degrees apart, in a three-phase converter the phases are 120 degrees apart, etc.). This phasing is useful because the input ripple current and the output ripple current typically have maximum reduction when the phases are added together symmetrically. As in the output current, the ripple current is reduced by half and the ripple frequency is twice that of the operating frequency when two phases are operated in parallel. Thus, smaller input filter capacitors and smaller output filter capacitors may be used for a given design.
Another feature of a multiphase converter is the improvement of the load transient response of the converter with each additional phase. Typically, a PWM operates at a frequency around 500 kHz. Some converter designs are approaching a 1 MHz operating frequency to improve transient response. Some transient specifications are approaching 60 amperes per microsecond transient response. From the position of the load, looking back into the DC-to-DC converter, a two-phase 500 kHz converter looks substantially the same as a single 1 MHz converter. Therefore, a four-phase 500 kHz converter has approximately the performance as 2 MHz converter. In general, more phases added symmetrically will have the benefits of increased load current, improved transient response, better distribution of the heat loss, less input ripple current, less output ripple current, and potentially improved reliability.
Multiphase converters require the desired phase relationship to be maintained between the various outputs of the converter. Some converter systems use a reference/slave arrangement where multiple pins are used between integrated circuits (ICs) to set up the multiphase solution. In a reference/slave arrangement, one IC is the reference and the remaining IC""s are the slaves. Slave ICs are coded to be placed in the proper phase relationship to the reference. In most cases, the ICs need a clock that runs at 4 to 8 times the reference clock frequency. The ICs include counters and decoders to produce the proper phasing from the clock. One exemplary system uses phase-lock loops between ICs to configure a multi-phase solution. Such ICs are very complex, and several pins are required for each IC to enable the IC to define a phase relationship with respect to the other ICs.
The present invention solves the foregoing problems and other problems by providing a single-phase synchronizing converter that is configured to automatically synchronize with other single-phase synchronizing converters on a peer-to-peer basis. In one embodiment, the synchronizing converter is configured as a DC-to-DC converter. Two or more synchronizing converters are operated in parallel to produce a multi-phase converter. In one embodiment, a common bus between the synchronizing converters includes a sync line and an open-collector type output with a common pull-up resistor. Phasing is automatic, and the phasing changes automatically as converters are added or removed. This automatic phasing is referred to herein as auto-interleaving synchronization.
For example, using the synchronizing converter, a three-phase converter can be initially configured for an existing processor. The three-phase converter can be quickly changed to a four-phase converter by adding another phase. Each time the system is powered up, the various converters arbitrate among themselves for phase position. Thus, the phasing positions are random, but the phasing is symmetrical regardless of the number of phases. In one embodiment, a hot-swappable single-phase module can be plugged into any location of a parallel multiphase bus to produce a common output voltage. Each time an additional module is plugged in (while power is on) the modules adjust their respective phases for phase symmetry. In one embodiment, each module shares a substantially equal portion of the output load current.
In one embodiment of an auto-interleaving multiphase switching converter, sensed voltages are provided to control the output currents of respective channels. The sensed voltages are derived from respective voltage waveforms applied to inputs of respective inductors in respective channels. A respective PWM circuit controls a switch coupled to the input of each inductor. The PWM circuit causes the switch to alternately connect the input of the inductor to a voltage source and to ground. As a result, the voltage waveform at the input of each inductor is a rectangular wave voltage with an amplitude approximately equal to the magnitude of the voltage source and with a duty cycle controlled by the PWM circuit. The sensed voltage is proportional to an average value of the voltage waveform at the input of the inductor and can be derived by lowpass filtering the input of the inductor. The sensed voltage is a DC value of the voltage waveform at the input of the inductor.
In one embodiment of an auto-interleaving multiphase switching converter, the sensed voltages are used to achieve forced current sharing. The output currents of respective channels are adjusted to be substantially identical by adjusting the PWM circuits of respective channels accordingly to achieve substantially identical sensed voltages in all the channels.
In one embodiment of an auto-interleaving multiphase switching converter, the same voltage source is supplied to each channel of the multiphase switching voltage converter. The sensed voltage is an average of the duty cycles of the voltage waveform at the input of each inductor. The duty cycle of the input of an inductor is the same as the duty cycle of the PWM signal being applied to the switch. Identical sensed voltages indicate that the duty cycles of the voltage waveforms at the inputs of respective inductors are substantially identical. Identical duty cycles applied to identical inductors result in identical output currents.
In one embodiment of an auto-interleaving multiphase switching converter, two or more voltage sources are supplied to the multiphase switching voltage converter to drive a common output. For example, a +5 volts DC voltage and a +12 volts DC voltage can supply current to a common load. The different voltage sources are processed by different channels of the multiphase switching voltage converter. Each voltage source is coupled to a different inductor input. The outputs of the inductors are electrically connected together to provide the common output.
Identical sensed voltages achieve forced current sharing between two or more voltage sources. In the case of two or more voltage sources, identical sensed voltages do not necessarily indicate identical duty cycles for the voltage waveforms at the inputs of respective inductors. The sensed voltage is also proportional to the value of the voltage source. For example, the duty cycle for the channel with the +12 volts DC voltage source is less than the duty cycle for the channel with the +5 volts DC voltage source when the respective sensed voltages are substantially identical. The sensed voltages represent the average voltages at the inputs of the respective inductors. Again, substantially identical inductors with substantially identical average voltages result in substantially identical output currents.
The auto-interleaving multiphase switching converter establishes forced current sharing by comparing the average sensed voltages to a reference voltage. The output voltage of the commonly-connected inductors is used as the reference voltage for all of the channels. Offset voltages are produced based on the differences between the respective sensed voltages and the reference voltage. The respective offset voltage is added to the output of a feedback amplifier to generate a control voltage which is used to adjust the duty cycle of the PWM signal being applied to the respective switches coupled to the input of the inductor. The use of the offset voltages forces the sensed voltages of respective channels to track the reference voltage.
The duty cycle ratios determine the output voltage level based on the level of the input voltage. The output voltage level is controlled through a feedback voltage, which is proportional to the output voltage of the multiphase switching converter. An error amplifier compares the feedback voltage to a reference voltage. A change in the feedback voltage indicates that a change in the total output current is desired to keep the output voltage level constant for a different load. The change is distributed evenly among the channels by changing the duty cycle ratios of all the channels in response to variations in the feedback voltage.
The sensed voltages of the present invention are advantageously derived at the input of the inductors. Compensation for variations of parameters in the PWM circuits, switches, and other control circuits in the multiphase switching converter is automatic to assure accurate current sharing. For example, the switches are typically implemented by MOSFETs. The ON resistances of the MOSFETs can vary by 30 to 40 percent, thereby varying the voltage waveforms applied to respective inductors. The variations appear in the sensed voltages and are compensated accordingly.
Accurate current sharing ensures that heat and component stresses are evenly distributed in the power conversion circuit, thereby improving reliability. Embodiments in accordance with the present invention achieve accurate current sharing among multiple channels of a switching converter without directly sensing the currents of respective channels, thereby reducing cost and power loss associated with sensing resistors typically used to sense current.
In one embodiment, an overlap detection circuit detects an overlap between output pulses produced by two synchronizing converters. In one embodiment, when an overlap is detected, a random phase shift is introduced to shift the phase of one or both of the overlapped channels to move their phase positions by different amounts and/or different directions. In one embodiment, the phase of the overlapped channels are shifted in different directions, by different amounts, or both. In one embodiment, a control circuit dithers (e.g., increases or decreases) a reference voltage setting for each overlapped channel by xc2x1x millivolts. In one embodiment, the amount of change is advantageously chosen to be sufficient to move the channel pulse by more than one pulse width when the integrating capacitor in the feedback of an integrating error amplifier is shorted out.